The international and widely used telex service basically consists of an interchange over long distances of typewritten texts. This service uses the existing communication network making it possible to send through this network a series of characters which include the capital letters of the alphabet, the ordinal numbers, some punctuation marks and certain special characters, which must be duly shaped into a format and interchanged according to a protocol established in the CITT V 10 norm and S.15 and V.1 recommendations.
According to the transmission protocol, the interchange of indicatives (known internationally as "answerbacks") at the beginning and at the end of each transmission, ensuring that the message was wholly transmitted to the correct addressee, is a norm. The format of transmission is based on the Baudot code, which uses digital words of 5 parallel bits, their interpretation depending on the sequence of transmission or on a bit incorporated to that effect.
The conventional transceiver of telex is a typewriter on whose keyboard the text is written, this text appearing printed in the distant teleprinter of the addressee, whose printer automatically types the text manually entered on the keyboard of the typewriter or distant telex of the sender. In addition, the telex instrument incorporates some devices to dial the addressees, to establish communication and to verify the identity (i.e. the "answerback") of the same and to code and transmit the texts, as well as receiving and decoding the texts transmitted to them through the same medium.
The increase in the availability and possibilities of acess by diverse types of users of certain types of computers becomes more evident every day, specifically in the case of those known as "personal computers". These devices are used more and more by professionals and small enterprises, as well as being employed as peripherals in sophisticated computer systems with access to sources of data.
The conventional telex transceivers are, as well as every type of office machine in general, relatively expensive instruments, so that their incorporation by a small enterprise, even if it is quite desirable, in most of the cases must be carefully evaluated as regards their cost.